Antenna



May 20, 1952 R. M. slLLlMAN ANTENNA Filed Jan. 7, 1946 /NVENTOR ROBERT M SML/MAN AT TORNE V Patented May 20, 1952 ANTENNA Robert M. Silliman, Silver Spring, Md., assignor to the United States of America as represented by the Secretary of War yApplication January 7, 1946, Serial No. 639,648

5 Claims. (Cl. Z50-33) This invention relatesl to' apparatus for the transmission and reception of electrical energy. More particularly it relates to conical antennas adapted for use at very high frequencies. E

The problem of making an antenna to operate at very high frequencies and to cover a very wide frequency range has presented problems to the prior art. This is especially true of antennas adapted for use on aircraft, since the smallness of the antenna used becomes a necessary and irnimportant factor. The electrical characteristics of an antenna are, among other things, ldependent upon the size and shape of that antenna. This. means-that when the physical size of an antenna is limited by the use to which it is put, the design for optimum electrical performance becomes a compromise between the physical size Y and optimum electrical performance. It is an object of the present invention to provide an antenna which will operate satisfactorily over a frequency range exceeding a three to one ratio.

It is also an-object of the present invention to provide anY antenna which represents the best compromise between physical size and electrical performance.

It is a further object of the present invention to provide a connection from a coaxial transmission line to the antenna which is capable of mechanically supporting the conical antenna structure.

Y It is another object of the present invention to provide a tapered connection from a coaxial transmission line to an antenna which does not introduce unwanted electrical reflections.

It is a further object of the present invention to provide a method of ruggedly mounting and solidly supporting a conical antenna on a moving craftwithout the introduction of unwanted electrical reflections.

Generally this invention comprises the making of an antenna, consisting of two conical portions having lbases of equal dimensions which are joined together, which is adapted to be mounted on structures, such as aircraft,-within a housing of insulating material. A coaxial transmission line conveys radio frequency energy to the antenna Vthrough the apex of the conical portion of greater altitude. This apex is electrically joined to the coaxial transmission line by use of a tapered. section of coaxial transmission line cornmonly known as the transformer. `The characteristic impedance of this transformer where it joins the apexis equal to the characteristic impedance of the; conical antenna at this apex and the characteristic impedance of'the transformer 2 where it joins the coaxial transmission line is equal to the characteristic impedance of that line. The transformer is filled with a solid dielectric material which lends rigidity and support to the conical antenna structure.

Other objects, features and advantages of this invention will suggest themselves to those skilled in the art and will become apparent from the following description of the invention taken in connection with the accompanying drawings in which:

Fig. 1 is a diagrammatic sketch of a conical antenna built in accordance with the principles of this invention;

Fig. 2 is a diagram showing the electrical characteristics of an antenna constructed in accordance with the principles of the present invention;

and

Fig. 3 is a diagram showing the characteristics of another antenna constructed in accordance with the principles of the present invention.

Referring more particularly to Fig. 1, the antenna and its associated structure are generally designated by I9. The structure may be more fully appreciated by visualizing it as a figure of revolution about its longitudinal axis A.-A. The housing I2 made of insulating material encloses the antenna II. Screws I3 fasten housing I2 to the skin I4 of the aircraft. The screw connection I5 fastens antenna II at its upper apex I6 to the housing I2.

Radio frequency energy is conveyed to antenna II through its lower apex I'I which is connected to coaxial transmission line IB by the coaxial y transmission line transformer I9. Theinner conductor 20 of transformer I9 is connected directly from coaxial transmission line I8 to apex I'I and outer conductor 2l of transformer I9 is electrically connected directly from coaxial transmission line I3 to the skin I4 of the aircraft, which is the ground potential.

Since the characteristic impedance of a coaxial transmission line is determined principally by the ratio of the outer diameter of the inner conductor to the inner diameter of the outer conductor the transformer'section I9 is constructed with a diameter ratio which will give a characteristic impedance equal to that of the coaxial transmission line I8 where it is connected to it and a characteristic impedance equal to the antenna I I where it is connected to it. Dielectric material 22 is placed between outer conductor 2l of transformer I9 and its inner conductor`20. The transformer section I9 prevents the introduction cf radio frequency reflections despite the placing of the dielectric material between its conductors. The primary function of the length of tapered dielectric material 22 is to aid in the mechanical support of antenna Il. This dielectric material in combination with screw connection l solidly supports and securely holds the antenna H in position.

Experiments have been conducted to determine the best compromise between the allowable physical size yand electrical operation of the antenna Il. The results obtained by experiment have been substantiated and it has been found that the angle 23 at apex I6 should be substantially 90 degrees and the angle 24 at apex I1 in the neighborhood of 60 degrees to 70 degrees.

Referring to Fig. 2, the driving point impedance of a conical antenna having anangle 23 equal to 90 degrees and an angle 24 equal to 60 degrees is plotted against the overall length in wavelengths of the antenna (the distance between the apexes of the two cones). The former' is plotted on the ordinate and the latter on the abscissa. The driving point impedance is divided into two components represented by curves and 3|. The curve 30 represents the reactive component of the impedance and the curve 3| represents the resistive component. 'I'he reactance isnegative when the curve falls below the base line. It is obvious from inspection of curves 30 and 3l that they are of material assistance in arriving at the best compromise between physical size and electrical operation for any given set of conditions.

Referring to Fig. 3 the driving point impedance of a conical antenna having an angle 23 equal to 90 degrees and an angle 24 equal to 70 degrees is pltted against the overall length in wavelengths of the antenna (the distance between the apexes of the two cones). As in Fig. 2 the driving point impedance is plotted on the ordinate and the overall length in wavelengths on the abscissa. The curve 40 represents the reactive component of the characteristic impedance and the curve 4i represents its resistive component. The reactance is negative when the curve falls below the base line. These curves are utilized in the same manner as the curves shown in Fig. 2.

While there has been here described what is at present considered to be the preferred embodiment of the invention, it will be obvious to those skilled in the art that various changes and modications may be made therein without departing from the scope of the invention.

What is claimed is:

1. A wide-band antenna comprising a pair of electrically conductive cones extending in opposite directions and mounted base-to-base with their axes colinear and electrically and mechanically connected at their bases, one of said cones having an apex angle of 90, the other of said cones having an apex angle of between 60 and 70, said cones having bases of the same area and being of different lengths, the distance between the apices of said cones being between substantially .3 of one wavelength and one wavelength of the frequencies in the operating frequency band, and a coaxial impedance transformer electrically connected to said other cone and serving as a mechanical support for said cones.

2. A wide band antenna comprising a pair of electrically conducting cones extending in opposite directions and mounted base-to-base with their axes colinear and electrically and mechanically connected at their bases, one of said cones having an apex angle of the other of said cones havingan apex angle of between 60 and 70, said cones having bases of the same area and being of different lengths, the distance between the apices of said cones being between substantially .3 of one wavelength and one wavelength of the frequencies in the operating frequency band, a coaxial transmission line for feeding ultra-high frequency energy to said cones, and impedance matching means coupling said line to the apex of one of said cones.

3. .Awide band antenna for operation at ultrahigh frequencies comprising a pair of electrically conducting cones mounted base-to-base with their axes colinear, said cones extending in opposite directions and having bases of the same area and Idifferent apex angles and being of different lengths, the distance between the apices of the cones being between substantially .3 of

one wavelength and one wavelength oi the frequencies inthe operating frequency band.

4. A wide band antenna for operation at ultrahigh frequencies comprising a pair of electrically conducting cones mounted base-to-base with their axes colinear and extending in opposite directions, said cones having bases of the same area and different apex angles and being of different lengths, the distance between the apices of said cones being between substantially .3 of one wavelength and one wavelength of the fre-y quencies in the operating frequency band, a coaxial transmission line for feeding ultra-high frequency energy to said cones, and impedance matching means coupling said line to the apex of one of said cones.

5. An antenna according to claim 4 in which said impedance matching means comprises linearly tapered inner and outer coaxial conductors, the ratio of inner-to-outer conductor diameter at the ends of said matching means being such that the impedances of said matching means at said ends are equal respectively to the impedance of said line and to that of said one cone.

ROBERT SILLIMAN.

REFERENCES CITED The following references are of record in thev nie of this patent:

UNITED STATES PATENTS Number Name Date 1,639,727 Fraser Aug. 23, 1927 2,237,778 Carter Apr. 8, 1941 2,267,889 Aubert` Dec. 30, 1941 2,275,646 Peterson 1 Mar. 10, 1942 2,313,046 Bruce Mar. 9, 1943 2,433,698 Hurst Dec. 30, 1947 FOREIGN PATENTS Number Country Date 114,368 Australia ..1 Dec. 24, 1941 469,366 Great Britain July 23, 1937 633,'148 France Jan. 21, 1928 

